Intensity spatial filter having non-uniformly spaced filter elements

ABSTRACT

A spatial filtering system for inspecting integrated circuit photomasks, and the like. The system employs a spatial filter comprising a matrix-like array of opaque regions on a transparent field. Unlike prior art systems where the region-to-region spacing of the filter is uniform, in the instant invention the region-to-region spacing steadily increases from the centermost element outward according to a precise mathematical formula.

United States Patent [1 1 Heinz et a1.

[54] INTENSITY SPATIAL FILTER HAVING NON-UNIFORMLY SPACED FILTERELEMENTS [75] Inventors: Robert Alfred Heinz, Flemington Township,Hunterdon County; Robert Charles Oehrle, Edgewater Park Township,Burlington County; Laurence Shrapnell Watkins, Hopewell Township, MercerCounty, all of N.J.; Terrence Edward Zavecz, Macungie Township, LehighCounty, Pa.

[73] Assignee: Western Electric Company,

Incorporated, New York, NY.

[22] Filed: May 3, 1972 [21] Appl. No.: 249,983

[52] US. Cl. 356/71, 350/162 SF, 356/239 [51] Int. CL; G0ln 21/32, G02b27/38 [58] Field of Search 350/162 SF, 3.5;

[5 6] References Cited UNITED STATES PATENTS Driver et a1. 350/162 SF3,738,752 June 12, 1973 3,630,596 12/1971 Watkins 350/162 SF 3,658,4204/1972 Axelrod 356/71 3,614,232 10/1971 Mathisen 356/71 OTHERPUBLICATIONS Watkins, Proc. of the IEEE, Vol. 57, N0. 9, September 1969,Pages 1634-1639.

Lohmann et 211., Applied Optics, Vol. 7, No. 4, April 1968, Pages1651-655.

Primary Examiner-David Schonberg Assistant Examiner-Ronald J. SternAtt0meyW. M. Kain, J. B. Hoofnagle, Jr., and J. L. Stavert 5 7] ABSTRACTA spatial filtering system for inspecting integrated circuit photomasks,and the like. The system employs a spatial filter comprising amatrix-like array of opaque regions on a transparent field. Unlike priorart systems where the region-to-region spacing of the filter is uniform,in the instant invention the region-to-region spacing steadily increasesfrom the centermost element outward according to a precise mathematicalformula.

4 Claims, 8 Drawing Figures PAIENIED JUN I 3. 738 752 SHEEIZBFS PAIENIEDJUN 1 21975 sum-3 or 3 A V ZEEO 20mm mozs'ma ORDER INTENSITY SPATIALFILTER HAVING NON-UNIFORMLY SPACED FILTER ELEMENTS BACKGROUND OF THEINVENTION 1. Field of the Invention Broadly speaking, this inventionrelates to spatial filtering. More particularly, in a preferredembodiment, this invention relates to an improved spatial filteringsystem which inhibits transmission of substantially all periodicinformation in the filtered image, thereby significantly improving thesignal-to-noise ratio of the system.

2. Discussion of the Prior Art As is well known, in the manufacture ofintegrated circuits, and the like, wafers of silicon, or othersemiconductor material, are coated with a layer of photoresist and,then, exposed to light through a special photographic plate, known inthe industry as a photomask. The exposed photoresist is then developed,in the conventional manner, and unexposed areas of the photoresist areremoved, thereby, exposing underlying portions of the silicon wafer.These exposed portions are then subjected to processing steps, such asdiffusion, etching, and the like.

A typical IC photomask may comprise a matrix-like array of thousands ofnominally identical photomask features, each in itself a complex patternof lines and other geometric shapes. Such photomasks have heretoforebeen made by successive photographic reductions from a large, hand-mademaster pattern, in a step-andrepeat camera, or. more recently, by directexposure of a photographic plate or chromium coated plate in acomputer-controlled electron beam machine. More recently still, aprimary pattern generator (PPG), a computer-controlled,electro-mechanical, laser deflection system, has been successfullyemployed to manufacture IC photomasks [See Bell Sygtem TechnicalJournal, (Nov. 1970), Vol. 49, No. 9, pages 2031-2076].

However, regardless of the manufacturing process employed, IC photomasksare expensive and time consuming to make. Accordingly, every effect ismade to prolong their useful life. Because of the extremely highresolution required with modern IC devices, exposureofphotor'esist-covered silicon wafers can only be satisfactorilyaccomplished by a contact-printing process, in which the emulsion sideof the photomask is placed in direct physical contact with the wafer.This frequently results in damage to the mask during exposure.Furthermore, pinhole defects may occur during manufacture of thephotomask itself, and dust or dirt may settle on the mask during use.

These defects are, ofcourse, very serious, for any wafer exposed tolight through a damaged or dirty photomask may yield dozens ofdefective, or wholly inoperative, IC devices. This situation is furtheraggravated by the fact that the same photomask is used over and overagain. Thus, a given defect on a mask might be responsible for thousandsof defective IC devices, a most undesirable situation.

As previously discussed, IC photomasks are too exr pensive to bediscarded after they have been used only a few times. Accordingly, itbecomes necessary to carefully inspect each mask after manufacture andalso, somewhat less critically, during actual production. Heretofore,these inspections were done manually by a skilled human operator, withthe aid of a microscope. However, because of the complex nature of thegeometric pattern in each photomask feature, as well as the fact thateach mask contains many thousands of identical features, human error andfatigue have been found to result in the failure to detect significantnumbers of defects.

To overcome this problem, a spatial filtering technique was developed toinspect the photomasks. This technique forms the subject matter ofcopending U. S. Pat. application, Ser. No. 858,002, filed Sept. 15,1969, (Watkins Case 1), which application is assigned to the assignee ofthe instant invention.

As disclosed in said copending application, the photomask to beinspected is illuminated by spatially coherent radiation from a laserand positioned proximate the front-focal plane of a convex lens. Inaccordance with well-known optical principles, an image will be formedat the rear-focal plane of the lens which corresponds to a Fouriertransform of the photomask. That is to say, the image is a compositediffraction pattern whose spatial distribution is the optical product oftwo components: (1) the interference function of the photomask,comprising a distribution of bright dots of light whose spacing isinversely proportional to the spacing between adjacent features in themask; and (2) the diffraction pattern of a single feature. Now, asdisclosed in said copending application, if a spatial filter comprisingan array of opaque regions on a transparent field, is positionedproximate the back-focal plane of the lens, and if the spacing betweenthe opaque regions corresponds exactly to the spacing between the dotsof light in the diffraction pattern, substantially all of the lightenergy from the laser will be blocked.

However, if the mask is defective in some way, for example, if the maskis scratched, etc., the Fourier transform of the defect will notspatially correspond to the pattern of opaque regions on the filter, andaccordingly, some light will succeed in passing through the filter,thereby enabling the scratch or other defect to be easily detected.

The above-described spatial filtering technique has been highlysuccessful in practice. However, certain problems were encountered whenan attempt was made to automate the inspection process. For example, inorder to eliminate the human factor, a television camera, coupled to acounting device, was positioned to view the filtered image of the mask.As the camera scanned over the image, the counting device recorded thenumber of defects detected, and, if the value so found exceeded somepredetermined value, the mask was discarded, or set aside for possiblerepair.

The system disclosed in copending application, Ser. No. 858,002,(Watkins Case I) assumed, for the sake of simplicity, that theinterference function produced by a lens comprises equally spaced dots.In practice, this is not exactly so, and the lens generates aninterference function in which the dots become progressively furtherapart by very small increments. Furthermore, the lens may suffer fromone or more optical aberrations, such as coma, astigmatism, fieldcurvature, and distortion. The net effect is that, as the light energyimpinges on those parts of the filter which lie further and further awayfrom the center of the filter, the opaque regions thereon no longerfully block the light which is coming from the photomask, even in thetotal absence of defects on the mask. This is so for two reasons: first,the outermost regions are improperly positioned to fully intercept thelight from the photomask, even if it were properly focused on theregions. Secondly, because the opaque regions are physically located ona planar surface, the outermost opaque regions lie increasingly a smalldistance apart from the true focus of the lens, and hence, in effect,become progressively too small to fully block the light from thephotomask. The outermost regions, of course, are intended to interceptthe higher spatial frequencies from the photomask and, in practice, theonly features on the mask possessing such higher spatial frequencies arethe edges of the photomask features.

In prior art systems, where the filtered image was inspected by a humanoperator, this failure to fully suppress periodic, high frequency, edgeinformation did not prove to be a significant problem. In fact, it wassomewhat of an advantage, because the outline of the individualphotomask features could be seen very faintly in the background of theimage, as viewed by the operator. Thus, the approximate location of thenonperiodic defects which were successfully isolated by the system couldbe rapidly ascertained. However, in an automated process, this no longerholds true, because a television camera does not have a human operatorsability to reason and is unable to discriminate between a true defectand the high frequency edge information of the photomask features. Thus,in the automated process, the edge information was erroneously countedas a defect, which it is not. An additional problem with the prior artapproach is that, because of the presence of high frequency edgeinformation, only a few of the thousands of features on a mask can beinspected at the same time.'Now, if an attempt is made to increase thefield of view, that is to say, if instead of inspecting only twenty orso of the thousands of features on a given mask, it is desired tosimultaneously examine several hundred features, the spatial filtermust, accordingly, be made with considerably more accuracy.

SUMMARY OF THE INVENTION As a solution to these and other problems, itis one object of this invention to provide a method of spatiallyfiltering an image which suppresses substantially all periodicinformation in the image, thereby significantly enhancing thesignal-to-noise ratio of the image.

It is a further object of this invention to provide a novel constructionfor a spatial filter to practice the above method.

Accordingly, one embodiment of the invention comprises a method ofisolating non-periodic errors in a two-dimensional pattern containing aregular array of nominally identical features, mutually spaced apart,along at least one axis, by a predetermined distance. The methodcomprises the steps of first directing a spatially coherent beam oflight at the pattern to diffract the light; and then focusing thediffracted light on a filter containing a plurality of discrete opaqueregions on a transparent field, the spacing between adjacent regions,along at least one axis of the filter, increasing from region-to-region,from the centermost region outwardly to the edges ofthe filter, tospatially modulate the light. Next, the spatially modulated light isreimaged to form an image exhibiting the non-periodic errors in thepattern, the filter blocking essentially all periodic information in theimage, including the higher spatial frequency components.

For practicing the above method, another embodiment of the inventioncomprises a spatial filter including a matrix-like array of opaqueelements on a transparent field, the spacing between adjacent elementsof the filter, along at least one axis of the array, increasing fromelement-to-element from the centermost element outwards to the edges ofthe array. Thus, when the filter is positioned to intercept the Fouriertransform of the image of a workpiece, for example, a workpiececomprising a matrix-like array of nominally identical features, theopaque elements act to inhibit further transmission of substantially allperiodic information in the Fourier transform.

In yet another embodiment of the invention, the distance X of any givenelement along said at least one axis, measured from the centermostelement on the filter, is given by the formula:

X=ftan [sin' (nA/d)] where,

f= the focal length of the Fourier transform lens;

A the wavelength of the light forming said image;

n the order of the spatial harmonic;

d the step-and-repeat of the workpiece.

The invention, and its mode of operation will be more fully understoodfrom the following detailed description, and the following drawing, inwhich:

DESCRIPTION OF THE DRAWING FIG. 1 is a partially schematic, isometricview of a first embodiment of the invention;

FIG. 2 illustrates a typical workpiece of the type which may beinspected by the instant invention;

FIG. 3 shows an enlarged view of a portion of the workpiece shown inFIG. 2;

FIG. 4 depicts the format of the diffraction pattern produced when theworkpiece of FIG. 2 is inspected by the apparatus of FIG. 1;

FIG. 5 depicts an illustrative prior art spatial filter;

FIG. 6 is a diagram illustrating the theory underlying the instantinvention;

FIG. 7 is a graph showing the spacing of filtering elements on thefilter of FIG. 5, as a function of the spatial harmonic; and

FIG. 8 depicts the relative orientation of the filtering elements of aprior art filter and the filter according to this invention.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 depicts an illustrativeembodiment of the invention. As shown, the apparatus comprises a laser10 which, when connected to a suitable source ofenergy (not shown),emits a beam of spatially coherent, radiant energy along a longitudinalaxis 1]. The light from laser 10 is directed through a beam expander 12,comprising a first lens 13 and pinhole 14. The expanded beam is thenpassed through a collimating lens 16 and finally falls upon the ICphotomask 17 to be inspected.

FIGS. 2 and 3 illustrate photomask 17 in greater detail. As shown, thephotomask comprises a glass photographic plate 18 having recordedthereon a matrix-like array of nominally identical features 19. As shownin FIG. 3, each feature comprises a complex pattern of opaque areas 21on a transparent field, the pattern in each feature defining the areasof the photoresistcovered semiconductor wafer which are to be protectedfrom exposure to the light. It will be noted that all of the edges ofthe areas 21 in feature 19 are parallel to either the horizontal or tothe vertical axes of the mask. By analogy to the orientation of theblocks in a typical city, such a configuration is frequently referred toas Manhattan geometry, although, of course, the invention is not limitedto inspecting workpieces having such Manhattan geometry, and can inspectwith equal success other workpiece configurations. It will also be notedthat, in FIG. 2, a uniform spacing D is assumed to exist between thecenter lines of each feature on the mask. It is further assumed thatthis spacing is the same in both the horizontal and vertical directions(i.e., D D). Occasionally, a photomask is produced in which thefeature-to-feature spacing differs in the horizontal and verticaldirections. However, this is easily compensated for in the design of thespatial filter, and the underlying theory of the instant inventionapplies to both arrangements.

In the drawing, mask 17 is depicted as having a 5 X 5 matrix of featuresthereon. One skilled in the art will appreciate that this is merely forconvenience in illustrating the invention and that an actual photomaskmay have as many as 40,000 features thereon arranged in a 200 X 200matrix.

Returning now to FIG. 1, photomask 17 is positioned at the front-focalplane of a second lens 22 which, as previously discussed, will form aFourier transform of the photomask at the back-focal plane thereof. Inaccordance with the teachings of copending application, Ser. No.858,002, (Watkins Case 1) spatial filter 23 is positioned at theback-focal plane to intercept all periodic information from photomask l7and to permit all non-periodic information, such as defects in thephotomask, to pass through the filter. The non-periodic informationwhich does succeed in passing through filter 23 is imaged by a thirdlens 24 for viewing by a television camera 25. As will be explainedbelow, camera 25 is connected by a lead 26 to a control circuit 27,which includes conventional power supplies, amplifiers, deflectionapparatus, etc. A digital-readout device 28 is connected to controlcircuit 27 by a lead 29 to record the number of defects in photomask 17which succeed in passing through spatial filter 23 and are detected bycamera 25.

FIG. 4 illustrates the pattern which would be seen if a screen were tobe positioned at the back-focal plane of lens 22', rather than spatialfilter 23. For convenience in drawing, this pattern is shown as a seriesof black dots on a white field. It will be appreciated that, in actualpractice, each of the black dots in FIG. 4 represents a spot of brightlight. As seen, the pattern approximates a cross with the spacingbetween adjacent light dots, in the horizontal direction, beinginversely proportional to the feature-to-feature spacing in thehorizontal direction in mask 17. Similarly, the spacing between adjacentdots, in the vertical direction, is inversely proportional to thefeature-to-feature spacing in photomask 17 in the vertical direction.If, as discussed, the feature-to-feature spacing on the mask is uniform,and equal, in both directions, then the dot-todot spacing in thediffraction pattern will also be uniform, and equal, in both directions.The large central dot 31 corresponds to the d.c. term of the Fouriertransform and, moving to the right, in the horizontal direction, dot 32corresponds to the first harmonic," or fundamental spatial frequency,(i.e., the step-andrepeat pattern of the mask), dot 33 the secondharmonic, and so on.

FIG. 5 depicts a spatial filter of the type disclosed in theabove-referenced copending application, Ser. No. 858,002, (Watkins Case1). This filter comprises an array of opaque regions on a transparentfield. This type of filter can be manufactured by the use of any ofseveral known techniques, in essentially the same manner that thephotomask itself may be manufactured. Considerable success has beenobtained by the use of the above-referenced primary pattern generator,and a step-and-repeat camera. If, as is usually the case, thefeature-to-feature spacing on the mask is uniform, and equal, along boththe horizontal and vertical axes, then the array of opaque regions inthe spatial filter will also be uniform, and equal, along both axes, andwill coincide with the location of light spots 31 through 34, etc., inFIG. 4.

While the intensity and size of the light dots in the actual diffractionpattern of FIG. 4 may vary, the opaque regions in FIG. 5 are typicallyall uniform in size and density. Of course, the regions must ,be largeenough to block the largest of the light dots shown in FIG. 4.

As previously discussed, the system described in copending application,Ser. No. 858,002, (Watkins Case 1), assumed that the lens was perfectand produced equally spaced dots, and this assumption was reasonable forthe inspection scheme contemplated by that invention. However, for morecritical applications, this assumption is not valid, and. the deviationsmust be taken into account. In FIG. 6 a lens 41 is shown positioned sothat a diffraction grating 42 is at its frontfocal plane. Thediffraction grating has elements spaced apart by a uniform distance d.Typical light rays 43 are shown coming from the diffraction grating atan angle 0 to the horizontal axis, as shown, and are imaged by lens 41onto the back-focal plane of lens 41.

' From basic diffraction theory, it is known that when a plane,collimated beam of light is incident upon an intensity grating, theresulting pattern behind the grating is the superimposition of manyplane waves, each propagating in a different direction. The angle 0 atwhich these beams emanate from the grating is a function of theharmonic, n, which they represent, that is:

sin 0=n Md where,

)t the wavelength of light;

n the order of the harmonic;

d the step-and-repeat of the array; and

f the focal length of the Fourier transform lens. Each of these waves isthen focused to a spot in the back-focal plane by the Fouriertransforming lens 41. The hemispherical surface 44 has been included toaid in computing the location of these images in the plane 45. Thelocation of the light spots on the plane 45 can then be computed fromsimple geometry:

Since for small angles, i.e., low spatial frequencies, sin 0 E tan 0 E0, the above equation reduces to the form which was assumed in theabove-referenced copending application, (Watkins Case 1), namely,

X n x /d FIG. 7 is a graph showing the distance from the origin (center)of the opaque filter regions, as a function of the order of the spatialfrequency, for the linear equation assumed in the copending application,and for the actual equation given in Equation 2 above. It will beobserved that for the first few orders, the deviation between the lineargraph and the actual, approximately tangential, graph is very small, buttowards the higher orders, this discrepancy becomes increasingly larger.

The upper half of FIG. 8 depicts the uniform regionto-region spacingemployed in prior art spatial filters, corresponding to the linear graph47 in FIG. 7. According to the invention, however, in the improvedspatial filter, the region-to-region spacing is not uniform butincreases according to curve 48 in FIG. 7. Thus, as shown in the lowerhalf of FIG. 8,'while the first few opaque regions in the filter are atapproximately the same position as they would be for the linear case,if, for example, one moves outward, to the right, from the center of thefilter, the discrepancy between the position of the regions in thelinear filter and those in the non-linear filter becomes increasinglylarge. Again, it must be emphasized that for clarity, the scale has beengreatly exaggerated.

Because the step-and-repeat spacing of typical integrated circuitdevices varies from 20 to 120 mils, the typical spacing between theopaque regions on a spatial filter varies from 20 to 120 microns,assuming an I-IeNe laser and a 100 mm focal length lens. It is,therefore, essential that the filter be manufactured with the greatestcare, and considerable accuracy is required to suecessively increase thedistance between the regions, in accordance with Equation 2. Thus, if aspatial filter, constructed in accordance with Equation 2 and graph 48of FIG. 7 is substituted for the spatial filter 23 in FIG. 1, the filterwill effectively block all periodic information from the photomask 17,including the edge information, even though the dots are actuallypositioned on planar surface 45, rather than the actual back plane oflens 22. The instant invention, of course, does not fully compensate forthe fact that light from the photomask is not completely focused on thespatial filter. However, this can be compensated for, if desired, bymaking the size of the regions for suppressing the higher spatialfrequencies slightly larger than those for suppressing the lower spatialfrequencies. From a practical standpoint, these requirements are sodemanding that production of a spatial filter, according to thisinvention, can only be effected in a computer-assisted device, such asthe PPG or a computer-controlled electron-beam machine.

One skilled in the art will appreciate that while the invention has beendescribed with reference to the inspection of integrated circuitphotomasks, it may also be used to inspect any workpiece having opticalcharacteristics approximating those of an optical grating, eithertransmissive or reflected, e.g., a processed silicon semiconductorslice. For example, the invention has successfully been used to inspectfine metallic grids, and diode array targets, such as those used in themanufacture of Picturephone camera tubes, and the like. Further, ifdesired, the spatial filter might comprise a matrix of transparentregions on an opaque LII field, rather than a matrix of opaque regionson a transparent field. ln this'latter event, periodic information wouldbe transmitted, rather than blocked. Of course, the term regions, asused herein, is intended to comprise various shapes, such as circles,squares, triangles, etc. The actual shape employed is merely a matter ofconvenience, provided that the corresponding light dot in thediffraction pattern is blocked. Also, various changes and substitutionsmay be made to the elements shown, without departing from the spirit andscope of the invention.

Finally, it must again be stressed, that while Manhattan geometry is byfar the most common found in integrated circuits, the methods andapparatus of this invention may be used to inspect workpieces having anygeometry in their features.

What is claimed is:

l. A method of isolating non-periodic errors in a twodimensional patterncontaining a regular array of nominally identical elements, mutuallyspaced apart along at least one axis by a predetermined distance, whichcomprises the steps of:

directing a spatially coherent beam of light at the pattern to diffractthe light;

focusing the diffracted light on a filter consisting of a plurality ofdiscrete substantially equally sized opaque regions on a transparentfield, the spacing between adjacent regions, along at least one axis ofthe filter, increasing from region-to-region from the centermost regionto the edges of the filter, to spatially modulate the light wherein thedistance X of any given region, from said centermost region, along saidat least one axis, is given by the formula:

where,

f the focal length of the Fourier transforming lens;

A the wavelength of said beam; n the order of the spatial harmonic; dthe step-and-repeat distance of said two- 7 dimensional pattern; and

reimaging the spatially modulated light to form an image exhibiting thenon-periodic errors in the pattern, said filter blocking essentially allperiodic information in said image, including higher spatial frequencycomponents.

2. Apparatus for inspecting non-periodic errors in a two-dimensionalpattern containing a plurality of nominally identical and regularlyspaced elements, arranged in a planar periodic array, which comprises:

means for direcging a spatially coherent beam of light at the plane ofthe pattern so that the light is diffracted thereby;

a first lens positioned to focus the light diffracted by the pattern;

a planar optical filter consisting of distribution of discretesubstantially equally sized opaque regions on a transparent field, thespacing between adjacent regions, along at least one axis of the filter,increasing from region-to-region from the centermost region to the edgesof the filter,

wherein the distance X of any given region, on said at least one axis,from said centermost region is given by the formula:

)t the wavelength of said beam;

n the order of the spatial harmonic;

d the step-and-repeat distance of said twodimensional pattern; thefilter being positioned at the focal plane of the first lens forspatially modulating the intensity of the light focused thereon by thefirst lens;

a second lens positioned to reimage the light transmitted by the filterto form a visual image of the non-periodic errors in the pattern of theimage display means; and

means for projecting the visual image onto the image display means.

3. Apparatus according to claim 2 wherein said image display means andsaid projecting means comprises:

a television camera focused on said visual image;

control means, connected to said camera, for supplying deflectionsignals and power to said camera,

' said camera scanning across said visual image todetect saidnon-periodic errors; and counting means, connected to the video outputof said camera, for counting the number of nonperiodic errors sodetected.

4. A spatial filter, which consists of:

a matrix-like array of discrete substantially equally sized opaqueelements on a transparent field, the spacing between adjacent elements,along at least one axis of the array, increasing from element-toelement,from the centermost element outwards towards the edges of the array,wherein the distance X of any given element on said at least one axis,from said centermost element, is given by the formula:

X =ftan [sin (n )t/d) wherein,

f the focal length of the Fourier transforming lens;

A the wavelength of the light forming said image;

n the order of the spatial harmonic;

d the step-and-repeat distance of the workpiece;

whereby, when said filter is positioned to intercept the Fouriertransform of the image of a workpiece comprising a matrix-like array ofnominally identical features, said opaque elements inhibit furthertransmission of substantially all periodic information in saidtransform.

- L-566-PT UNITED STATES PATENT OFFICE CERTIFICATE OF CQRRECTION PatentNo. 3,73 ,75 Dated June 973 Imam) R. A. HEINZ, R. c. OEHRLE, L. s.WATKINS, T. E. ZAVECZ It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

In the specification, Column 1, line M1, "effect" should read --effort-.

In the claims, Claim 2, column 8, line E L, "direcging" should read--directing--; line 59, "of distribution" should read "of adistribution-. 1

Signed and sealed this 26th da of March 19m.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. C. MARSHALL DANN Atte sting Officer Commissionerof Patents

1. A method of isolating non-periodic errors in a twodimensional patterncontaining a regular array of nominally identical elements, mutuallyspaced apart along at least one axis by a predetermined distance, whichcomprises the steps of: directing a spatially coherent beam of light atthe pattern to diffract the light; focusing the diffracted light on afilter consisting of a plurality of discrete substantially equally sizedopaque regions on a transparent field, the spacing between adjacentregions, along at least one axis of the filter, increasing fromregion-to-region from the centermost region to the edges of the filter,to spatially modulate the light wherein the distance X of any givenregion, from said centermost region, along said at least one axis, isgiven by the formula: X f tan (sin 1 (n lambda /d) ) where, f the focallength of the Fourier transforming lens; lambda the wavelength of saidbeam; n the order of the spatial harmonic; d the step-and-repeatdistance of said two-dimensional pattern; and reimaging the spatiallymodulated light to form an image exhibiting the non-periodic errors inthe pattern, said filter blocking essentially all periodic informationin said image, including higher spatial frequency components. 2.Apparatus for inspecting non-periodic errors in a two-dimensionalpattern containing a plurality of nominally identical and regularlyspaced elements, arranged in a planar periodic array, which comprises:means for direcging a spatially coherent beam of lighT at the plane ofthe pattern so that the light is diffracted thereby; a first lenspositioned to focus the light diffracted by the pattern; a planaroptical filter consisting of distribution of discrete substantiallyequally sized opaque regions on a transparent field, the spacing betweenadjacent regions, along at least one axis of the filter, increasing fromregion-to-region from the centermost region to the edges of the filter,wherein the distance X of any given region, on said at least one axis,from said centermost region is given by the formula: X f tan (sin 1 (nlambda /d) ) where, f the focal length of the Fourier transforming lens;lambda the wavelength of said beam; n the order of the spatial harmonic;d the step-and-repeat distance of said two-dimensional pattern; thefilter being positioned at the focal plane of the first lens forspatially modulating the intensity of the light focused thereon by thefirst lens; a second lens positioned to reimage the light transmitted bythe filter to form a visual image of the non-periodic errors in thepattern of the image display means; and means for projecting the visualimage onto the image display means.
 3. Apparatus according to claim 2wherein said image display means and said projecting means comprises: atelevision camera focused on said visual image; control means, connectedto said camera, for supplying deflection signals and power to saidcamera, said camera scanning across said visual image to detect saidnon-periodic errors; and counting means, connected to the video outputof said camera, for counting the number of non-periodic errors sodetected.
 4. A spatial filter, which consists of: a matrix-like array ofdiscrete substantially equally sized opaque elements on a transparentfield, the spacing between adjacent elements, along at least one axis ofthe array, increasing from element-to-element, from the centermostelement outwards towards the edges of the array, wherein the distance Xof any given element on said at least one axis, from said centermostelement, is given by the formula: X f tan (sin 1 (n lambda /d) )wherein, f the focal length of the Fourier transforming lens; lambda thewavelength of the light forming said image; n the order of the spatialharmonic; d the step-and-repeat distance of the workpiece; whereby, whensaid filter is positioned to intercept the Fourier transform of theimage of a workpiece comprising a matrix-like array of nominallyidentical features, said opaque elements inhibit further transmission ofsubstantially all periodic information in said transform.